The electromagnetic coupling between a planet and its host star is commonly referred to as magnetic Star-Planet Interaction and in the following abbreviated as SPI. In that situation, the star creates a magnetised plasma environment for the planet through the stellar wind and magnetic field [1]}, [2]}. On its orbit, the planet moves through the plasma and magnetic field and excites different plasma waves. If the motion is partially perpendicular to the magnetic field, the planet excites Alfvén waves, which are the most important waves in SPI [3]}, [4]}. In the case that the planet orbits sufficiently close to its star that the wave speed exceeds the local plasma speed then the waves can travel upstream towards the star and establish a coupling [5]}, [4]}, [2]}. This situation is commonly referred to as sub-Alfvénic interaction [3]}. The principal process is well known from the moon-magnetosphere interaction in our Solar System.
